1. Field of the Invention
The present invention relates to a light source device that includes a discharge tube with a discharge medium sealed therein and electrodes for exciting the discharge medium, and relates to a liquid crystal display device employing the light source device.
2. Related Background Art
Recently, with respect to backlights (light source devices) for use in liquid crystal display devices, etc., studies have been directed to backlights in which mercury is not used (a backlight of this type hereinafter sometimes is referred to as mercuryless backlight), in addition to the studies on backlights in which mercury is used. The mercuryless backlight, which does not employ mercury, does not suffer from a decrease in a light emission efficiency caused by a rise of mercury temperature, thereby having an advantage of quicker start of the emission of luminous fluxes. Further, the mercuryless backlight is environmentally preferable.
As a light source device in which mercury is not used, a discharge lamp device is disclosed that includes a bulb in which a rare gas is sealed, an inner electrode provided inside the bulb, and an outer electrode provided outside the bulb (JP 5(1993)-29085 A). The outer electrode is a linear electrode, and is formed on an outer surface of the bulb so as to be parallel with a central axis of the bulb. By applying a voltage across the inner and outer electrodes, this rare gas discharge lamp device emits light.
Further, a rare gas discharge lamp is disclosed that includes a discharge tube in which a rare gas is sealed, an inner electrode formed inside the discharge tube, and an outer electrode formed spirally on an outer surface of the discharge tube (JP 10(1998)-112290 A).
Furthermore, as a discharge lamp with a rare gas as a principal discharge medium, a discharge lamp is disclosed that includes an air-tight vessel, an inner electrode provided inside the air-tight vessel, and an outer electrode in such a form as a coil form, a mesh form, etc. (JP 2001-325919 A). This publication discloses a method of fixing the outer electrode using a shrinkable tube.
Furthermore, a discharge lamp disclosed in U.S. Pat. No, 5,604,410 includes a discharge tube in which a rare gas is sealed, an inner electrode, and an outer electrode. The inner electrode is formed along a central axis of the discharge tube throughout a substantial entirety of the discharge tube. The outer electrode is a linear electrode, and is formed on an outer surface of the discharge tube so as to be parallel with a central axis of the discharge tube.
However, in the case where a linear outer electrode is formed throughout a substantial entirety of the discharge tube, the discharge is constricted in the vicinity of the outer electrode sometimes, thereby making it impossible to excite the discharge medium efficiently, sometimes resulting in a decrease in the light emission efficiency. In the case where an outer electrode in a spiral form is provided on an outer surface of a discharge tube, the discharge also tends to be constricted, since the outer electrode is brought into contact linearly throughout the entirety of the discharge tube.